Method for producing ethylene homopolymers and copolymers by means of radical high pressure polymerization

ABSTRACT

The invention relates to a process for preparing ethylene homopolymers and ethylene copolymers in a tube reactor at pressures above 1000 bar and temperatures in the range from 120 to 400° C. by free-radical addition polymerization, in which first small amounts of free-radical chain initiator are supplied to a streaming flow medium comprising ethylene, molar mass regulator, and, if desired, polyethylene, after which the polymerization takes place. The invention takes place at a pressure in the range from 2000 to 3500 bar and comprises reaction mixture within the tube reactor passing through a temperature profile in the range from 100 to 350° C.

[0001] Polyethylene is prepared by polymerizing ethylene by one of twofundamentally different methods, the high-pressure process and thelow-pressure/medium-pressure process. The low-pressure/medium-pressureprocess can be conducted as a solution polymerization, asuspension/emulsion polymerization or a gas-phase polymerization. Thehigh-pressure process is conducted at pressures above 1500 bar(corresponding to 150-400 MPa) and proceeds by a free-radical mechanism.

[0002] Generally speaking, the low-pressure/medium-pressure process isconducted at pressures below 100 bar, and is generally catalyzed. Incontrast to the products of the high-pressure process, which exhibit ahigh level of branching, relatively low crystallinity, and low density,products from the low-pressure/medium-pressure process are usually of alinear structure with little branching and possess a high level ofcrystallinity (generally 60-90%), a high melting range (typically120-135° C.), and a high density (generally 0.93-0.97 g/cm³). Highdensity in a polyethylene is normally associated at the same time with ahigh glass transition temperature, high hardness, a high melting range,high brittleness, and low tackiness. The above properties generallycharacterize low-pressure/medium-pressure polyethylene.

[0003] Since, however, from the applications standpoint these propertiesare not always required in every case, the low-density polyethylene(LDPE) prepared by the high-pressure process in tube reactors continuesto be a standard polymer for which a very high level of demand is stillbeing recorded worldwide, since the material is less brittle and fornumerous applications is simply easier to process.

[0004] For this reason, the high-pressure polymerization processcontinues to be an established process for preparing low-densitypolyethylene (LDPE), and is carried out industrially with great successin numerous plants worldwide. In the case of high-pressurepolymerization, the reaction is normally initiated by atmospheric oxygenor by peroxides or by other free-radical initiators or by mixtures ofthese. For this purpose, the free-radical chain initiators used toinitiate the polymerization must be added to the reaction medium in someappropriate way.

[0005] The text below, therefore, will consider the high-pressurepolymerization of ethylene where use is also made of comonomers such asvinyl acetate, vinyl esters, olefinically unsaturated carboxylic acidsor alpha-olefins. This is a copolymerization reaction in which thecrystallinity of the resulting product can be controlled specifically byvarying the amount of comonomer in the monomer mixture. The Uhde GmbHbrochure “engineering news 4-94” presents schematically under the title“The advanced Ruhrchemie Process” a process in which fresh ethylene isfirst subjected to low-pressure compression and then, together withrecyclate, which has not been consumed in the actual polymerizationreaction, with initiators, moderators, and comonomers, is brought toreaction pressure in a high-pressure compressor. The actualpolymerization reaction then takes place in a tube reactor, which isconstructed as a double-tube heat exchanger.

[0006] The reaction gas is first heated to a temperature in the rangefrom 90 to 200° C. in order to initiate the highly exothermicpolymerization reaction. The heat of reaction liberated is taken off bywater cooling, with up to 40% of the monomers used being converted topolymer in one pass through the continuously operated reactor.

[0007] The polymer formed is deposited in a two-stage process comprisinga high-pressure separation and a low-pressure separation. First of all,by a reduction in pressure to about 180-350 bar, unused reaction gas isvery largely separated off from the polymer which has formed, and, aftercooling and purification, is recycled to the polymerization reaction (asthe abovementioned recyclate). The polymer arriving at the low-pressureseparation stage is then freed from the remainder of the unreactedreaction gases by further reduction in pressure to 1-5 bar. The gasesare passed back again, while the resulting polymer is supplied to a meltextruder, homogenized, and then pelletized.

[0008] A disadvantage, however, is that the polymer prepared in this wayhas a certain tendency to form gel specks.

[0009] It is an object of the present invention, therefore, to conductthe high-pressure polymerization of ethylene and further comonomers insuch a way that, by optimizing the way in which the reactor is cooled,the reaction temperature, and the distribution of cooling water, agreater throughput is achieved without detriment to the quality of thepolymer thus prepared.

[0010] We have found that this object is achieved by a process for thehigh-pressure polymerization of ethylene in a tube reactor, constructedas a double-tube heat exchanger, in which fresh ethylene is firstsubjected to low-pressure compression and then, together with recyclate,initiators, moderators, and comonomer, is brought to reaction pressurein a high-pressure compressor, which comprises conducting thepolymerization at a pressure in the range from 2000 to 3500 bar andcomprises reaction mixture within the tube reactor passing through atemperature profile in the range from 100 to 35020 C.

[0011] The reaction mixture, comprising inert gas, molar mass regulator,ethylene, and comonomer, is preferably first compressed to a pressure of2800 bar and then heated to a temperature of 120° C. The hot reactionmixture is then fed into the tube reactor, in which, right at thebeginning, the peroxide initiator is added in an amount of from 10 to400 ppm, based on the weight of the reaction mixture, by way ofhigh-pressure piston pumps. The exothermic polymerization reaction,which begins immediately, liberates heat of reaction which must berapidly taken off by water cooling, since otherwise the reaction mixturewould suffer excessive overheating and there would be a danger ofuncontrolled decomposition of the ethylene. In accordance with theinvention, cooling is conducted such that the reactor as a whole isdivided lengthwise into two zones. The first zone encompasses the fronttwo thirds of the overall tube reactor length, the second zone the lastthird of the overall tube reactor length. Both zones of the reactor arecooled separately with water at different temperatures, the preferredwater temperature for the preparation of ethylene homopolymer in the twozones being calculated in accordance with the following formula:

T(H₂O) [° C.] zone 1=200−7.77·MFR

and T(H₂O) [° C.] zone 2=159−7.62·MFR,

[0012] MFR denoting the melt index of the product obtained at the end ofthe reactor, in [dg/min], measured in accordance with ASTM D-1238,Condition (E), i.e., at a temperature of 190° C. under an applied weightof 2.16 kg.

[0013] The inventively preferred water temperature for the preparationof ethylene copolymer with vinyl acetate in the two zones is calculatedaccording to the following formula:

T(H₂O) [° C.] zone 1=130−1.77·MFR

and T(H₂O) [° C.] zone 2=120−3.0·MFR,

[0014] MFR likewise denoting the melt index of the product obtained atthe end of the reactor, in [dg/min], measured in accordance with ASTMD-1238, Condition (E), i.e., at a temperature of 190° C. under anapplied weight of 2.16 kg.

[0015] At the end of the tube reactor the reaction mixture is cooled andsubstantially freed from volatile constituents by lowering the pressureto a figure in the range between 10 to 70 MPa. Thereafter the polymermelt is freed from the remaining adhering reaction gases as describedabove, by further pressure reduction to 1-5 bar, and is homogenized inan extruder, pelletized, and packed.

[0016] Surprisingly it has been found that, by observing the inventivelyprescribed temperature profile during the polymerization, heat transferis improved, and that the reactor yield can be increased as a result.

[0017] As molar mass regulators it is possible in accordance with theinvention to use customary polar or apolar organic compounds such asketones, aldehydes, alkanes or alkenes having 3 to 20 carbon atoms.Preferred molar mass regulators are acetone, methyl ethyl ketone,propionaldehyde, propane, propene, butane, butene, and hexene.

[0018] As free-radical chain initiators it is possible inventively touse peroxides such as aliphatic diacyl (C₃ to C₁₂) peroxides, tert-butylperoxypivalate (TBPP), tert-butyl peroxy-3,5,5-trimethylhexanoate(TBPIN), di-tert-butyl peroxide (DTBP), tert-butyl perisononoate ormixtures or solutions of these in suitable solvents. The free-radicalchain initiators are supplied inventively in amounts in the range from10 to 1000 g/t of PE produced, preferably from 100 to 600 g/t of PEproduced.

[0019] The streaming flow medium to which the abovementionedfree-radical initiators are supplied in accordance with the inventionmay include, in addition to ethylene, 1-olefin comonomer(s) having 3 to20 carbon atoms, preferably having 3 to 10 carbon atoms, in an amount inthe range from 0 to 10% by weight, based on the amount of ethylenemonomer, preferably in an amount in the range from 1 to 5% by weight.Additionally, the streaming flow medium may in accordance with theinvention comprise polyethylene in an amount in the range from 0 to 40%by weight, based on the total weight of the monomers, preferably from 1to 30% by weight.

[0020] In one particularly preferred variant of the process of theinvention the free-radical chain initiators are supplied to a region ofthe tube reactor in which by virtue of a reduction in the diameter ofthe tube reactor to a level of approximately 0.6 to 0.9 times thediameter D of the reactor in the feed zone the flow rate of thestreaming flow medium is increased to from 1.2 to 2.8 times, preferablyfrom 1.8 to 2.5 times, the flow rate within the feed zone of the tubereactor. Expressed in absolute figures, the flow rate of the streamingflow medium in the region of supply of the free-radical chain initiatorsis situated inventively in the range from 10 to 40 m/s, preferably from15 to 30 m/s, more preferably from 20 to 25 m/s.

[0021] The process of the invention has the advantage that stablereactor operation can be maintained at unusually high maximumtemperatures of up to 350° C. without any inclination towarddecomposition.

[0022] A further advantage of the process of the invention can be seenin the fact that the polymerization is initiated at lower temperaturesand that thereafter the temperature rise of the reaction mixture takesplace in a controlled manner. This allows better exploitation, for thepolymerization and hence for the preparation of LDPE, of the lifetime ofthe free-radical chain initiators, which commonly have only a relativelyshort half-life.

[0023] Practical tests conducted show that, as a result of the processof the invention, the conversion and, in particular, the productproperties such as density and MFR are improved. In accordance with theinvention it was possible to lower the amount of free-radical chaininitiator used by about 15%, and the consistency of operation of thetube reactor was increased.

EXAMPLE

[0024] In a tube reactor with an L/D ratio of 30 000 an amount of 28.5ppm of di-tert-butyl peroxy-2-ethylhexanoate and 200 ppm of hexane(solvent) were injected at the reactor entry, after heating to atemperature of 150° C., into a mixture of 98% by volume ethylene, 1.5%by volume propylene as moderator and 1.5 ppm oxygen. In the firstreaction zone the temperature of the cooling water was set at 195° C.,while in the second reaction zone the water temperature was reduced to154° C. by further addition of cooling water. The pressure within thereactor was 233 MPa (=2330 bar).

[0025] The polyethylene, worked up conventionally at the end of thereactor, had a density of 0.924 g/cm³ and an MFR of 0.65 dg/min.

1. A process for preparing ethylene homopolymers and ethylene copolymersin a tube reactor by free-radical addition polymerization, in whichfirst small amounts of free-radical chain initiator are supplied to astreaming flow medium comprising ethylene, molar mass regulator, and, ifdesired, polyethylene, after which the polymerization is then carriedout, in which the polymerization is conducted at a pressure in the rangefrom 2000 to 3500 bar and in which the reaction mixture within the tubereactor passes through a temperature profile in the range from 100 to350° C., wherein the heat of reaction, liberated immediately, is takenoff by water cooling, and wherein the water cooling is set up, for thepreparation of ethylene homopolymer, in two zones, of which zone 1encompasses the front two thirds of the total length of the tube reactorand of which zone 2 encompasses the last third of the total length ofthe tube reactor, the preferred water temperature for the cooling beingcalculated in accordance with the following formula: T(H₂O) [° C.] zone1=200−7.77·MFR and T(H₂O) [° C.] zone 2=159−7.62·MFR, MFR denoting themelt index of the homopolymer obtained at the end of the reactor, in[dg/min], measured in accordance with ASTM D-1238, Condition (E).
 2. Aprocess as claimed in claim 1, wherein the water cooling is set up, forthe preparation of ethylene copolymer, in two zones, of which zone 1encompasses the front two thirds of the total length of the tube reactorand of which zone 2 encompasses the last third of the total length ofthe tube reactor, the preferred water temperature for the cooling beingcalculated in accordance with the following formula: T(H₂O) [° C.] zone1=130−1.77·MFR and T(H₂O) [° C.] zone 2=120−3.0·MFR, MFR denoting themelt index of the copolymer obtained at the end of the reactor, in[dg/min], measured in accordance with ASTM D-1238, Condition (E).
 3. Aprocess as claimed in claim 1, wherein the reaction mixture, comprisinginert gas, molar mass regulator, ethylene, and comonomer, is firstcompressed to a pressure of 2800 bar and then heated to a temperature of120° C.
 4. A process as claimed in claim 3, wherein the hot reactionmixture is then fed into the tube reactor, in which, immediately,peroxide initiator is added in an amount of from 10 to 400 ppm, based onthe weight of the reaction mixture, by way of high-pressure pistonpumps.
 5. A process as claimed in claim 1, wherein following itsemergence from the reactor the polymer melt is freed from unreactedreaction gases by gradual pressure release, and is pelletized andpacked.
 6. A process as claimed in claim 1, wherein molar massregulators used are polar or apolar organic compounds such as ketones,aldehydes, alkanes or alkenes having 3 to 20 carbon atoms, preferablyacetone, methyl ethyl ketone, propionaldehyde, propane, propene,mbutane, butene or hexene.
 7. A process as claimed in claim 1, whereinas free-radical chain initiators peroxides such as tert-butylperoxypivalate (TBPP), tert-butyl peroxy-3,5,5-trimethylhexanoate(TBPIN), di-tert-butyl peroxide (DTBP), tert-butyl perisononoate ormixtures or solutions of these in suitable solvents are supplied inamounts in the range from 10 to 1000 g/t of PE produced, preferably from100 to 600 g/t of PE.